Confocal wafer inspection method and apparatus using fly lens arrangement

ABSTRACT

A semiconductor wafer inspection system and method is provided which uses a multiple element arrangement, such as an offset fly lens array. The preferred embodiment uses a laser to transmit light energy toward a beam expander, which expands the light energy to create an illumination field. An offset fly lens array converts light energy from the illumination field into an offset pattern of illumination spots. A lensing arrangement, including a first lens, a transmitter/reflector, an objective, and a Mag tube imparts light energy onto the specimen and passes the light energy toward a pinhole mask. The pinhole mask is mechanically aligned with the offset fly lens array. Light energy passing through each pinhole in the pinhole mask is directed toward a relay lens, which guides light energy onto a sensor. The offset fly lens array corresponds to the pinhole mask. The offset pattern of the offset fly lens array is chosen such that spots produced can be recombined into a continuous image, and the system utilizes a time delay and integration charge coupled device for rapid sensing along with an autofocus system that measures and cancels topological features of the specimen.

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/125,568, filed on Mar. 23, 1999,pursuant to 35 U.S.C. Sections 111 and 119(e).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to precision optical inspectionmethods for specimens such as semiconductor wafers, and morespecifically to a method and apparatus for performing microscopicinspection and measurement of integrated circuit wafer geometries usinglaser confocal microscopy.

2. Description of the Related Art

In integrated circuit inspection, particularly inspection ofsemiconductor wafers or photomasks from which such circuits arefabricated, different methods have been employed to address particularcharacteristics of the wafers or advantages afforded by specificinspection technologies.

One such method that has previously been employed in semiconductor waferor photomask inspection is confocal microscopy. Confocal imaging entailssuppressing out of focus specimen elements at image formation. Thesuppression of out of focus elements occurs partially as a result of thespecimen not being illuminated and imaged as a whole at one time, but asone point after another, and also due to the detection pinhole, orspatial filter, interposed between the source and specimen. Thesequential point imaging in confocal microscopy is obtained using anarrangement of diaphragms which act as both a point source and a pointdetector simultaneously at optically coexistant points of the path oflight rays used to inspect the specimen. Rays which are out of focus aresuppressed by the detection pinhole.

Other inspection techniques have been employed with varying results.Non-confocal imaging tends to be highly sensitive to signals locatedoutside the focal plane, which can add unwanted noise to the imagingprocess. Noise may also result from defect detection on a semiconductorwafer due to the various layers present on the wafer. Confocal imagingusing light elements other than lasers have been employed, but sucharrangements create illumination spots with inefficient and slowpinholes since the arc or filament cannot increase in intensity, due tolimited power.

Laser confocal imaging addresses the drawbacks of these previoussystems. An illustration of a typical laser confocal microscopyinspection arrangement for imaging a single point is illustrated inFIG. 1. Laser 101 emits a beam of light rays 102 which passes through afocusing lens 103 and subsequently through first spatial filter 104.After passing through confocal element 104 the light rays flare outwardtoward beamsplitter 105. Beamsplitter 105 allows the light rays to passthrough and toward objective 106, which focuses the light rays towardthe specimen. Light rays reflected from an object at the focal plane 107pass back toward the objective 106 and beamsplitter 105. Beamsplitter105 at this point reflects the light rays as illustrated toward secondconfocal element 108. Objects above or below the focal plane 107 are outof focus and are therefore suppressed. The remaining in focus light rayspass to detector 109.

The advantages of confocal microscopy include the feature that lightrays from outside the focal plane are not registered. Confocal imagingcan provide true three dimensional data recording, but instead graduallyoptically removes portions of the specimen as those portions move awayfrom the focal plane. In practice, the elements tend to disappear fromthe field of view. Stray light tends to be minimized in a confocalarrangement.

The drawbacks associated with the arrangement of FIG. 1 are that thesystem shown therein only of confocal imaging include a limited field ofview, typically a small point on the specimen. Thus scanning an entirespecimen would require several passes even for moderately sizedspecimens. Speed and throughput tend to be of great importance duringwafer inspection, and thus confocal techniques have been limited intheir application.

Multiple scanning spot systems have been employed to increasedinspection speed and throughput. These multiple scanning spot systemsutilize mechanical polygon scanning spot laser arrangements to provideincreased scanning areas. However, the mechanical polygon scanningtechniques tend to be highly unstable and do riot provide necessary fineimage alignment for comparison of adjacent features under mostcircumstances.

Certain confocal systems employ techniques for performing inspection ofa wafer or specimen but each system has particular negative aspects. Forexample, U.S. Pat. No. 5,248,876 to Kerstens, et al., illustrates aconfocal imaging system using an opaque mask having a slit and a row ofpinpoint sensors or a skewed pattern of isolated pinholes with an arrayof isolated pinpoint sensors in a matching pattern. The problem withsuch an arrangement is the sensing of data. The Kerstens sensingarrangement employs an array 116 having isolated pinpoint radiationsensors 114. The problem with such a system is that it is inherentlyslow and, inefficient in scanning large amounts of data. In particular,the Kerstens system has a very limited dynamic range and can result inobscured or saturated parts of the image under normal inspection speeds.

The Kerstens system also uses a type of autofocus system which usesmultiple confocal measurements to determine features on the surface ofthe system. In particular, the effective focus position of the Kerstenssystem is a function of the position on the wafer such that the geometryeffects the ability of the system to focus on a particular feature andmeasuring the height of a particular feature.

Other known confocal inspection systems can have problems maintainingfocus on a single layer in a multiple layer specimen, such as a CMP(Chemical Mechanical Planarization) specimen. On a multiple layerspecimen, inspection of the topmost surface may be required, and certainsystems employing confocal techniques do not provide the ability todiscriminate or focus on the desired layer. Focusing becomes a problemdue to decreasing line widths, desire for increased optical resolution,and a corresponding decrease in depth of focus. Most autofocus systems,such as the autofocus system presented in the Kerstens reference, seethrough the multiple layers on specimens such as CMP specimens to followthe underlying layers, resulting in non-planar focus performance havingvarying sensitivity to surface defects resulting from following theunderlying topology.

It is therefore an object of the current invention to provide aninspection system which provides noise reduction in images received andthe ability to control focus in a multiple depth environment, such as aCMP specimen, in particular with respect to signals which are out of thedepth of focus and out of the depth of application interest of thesystem.

It is a further object of the current invention to provide a high speedand accurate brightfield and darkfield image inspection system withoutthe speed, illumination, and processing drawbacks of traditional laserinspection systems or non-laser systems utilizing lamps.

It is another object of the current invention to provide an inspectionsystem which minimizes the instabilities associated with mechanicalscanning systems.

It is yet another object of the current system to provide a confocalsystem having the ability to inspect a comparatively large area at arelatively rapid rate with minimal distortion at a maximum dynamicrange.

It is a further object of the current system to provide for a robust andeffective focusing system for multiple layer specimens that is simpleand has the ability to effectively discern and account for heightdifferences within the specimen.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a semiconductorwafer inspection system and method using a multiple element arrangement,such as an offset fly lens array. The preferred embodiment of thecurrent system uses a laser to transmit light energy toward a beamexpander, which expands the light energy from the laser beam to createan illumination field. The system uses an offset fly lens array toconvert the light energy from the illumination field into an offsetpattern of illumination spots. The offset pattern of illumination spotsis transmitted to a first lens which focuses the array of light energyproduced toward the surface of the wafer specimen. Light energy thenpasses through a transmitter/reflector which transmits the light energytoward the surface of the wafer specimen. Light energy from thetransmitter/reflector passes through objective which provides an arrayof beams, rather than a single beam, onto the surface of the waferspecimen, and the image from the wafer surface is reflected back throughthe objective. Light energy passes toward transmitter/reflector, whichreflects light energy toward a tube lens, which focuses the multiplebeams toward a pinhole mask. The pinhole mask is mechanically alignedwith the offset fly lens array according to an approximate tolerance.Light energy passing through each of the pinholes in the pinhole mask isdirected toward a relay lens, which directs light energy onto a sensor,preferably a TDI CCD sensor. Other sensors may be used which permitreceipt of light energy.

The offset fly lens array is preferably a 16 (X direction) by 128 (Ydirection) element lens array, as is the corresponding pinhole mask. Theoffset pattern of the offset fly lens array is chosen and arranged suchthat the spots produced from the offset fly lens array can be recombinedin the system into a continuous image by image sensor. Binary optics maybe employed to create a darkfield illumination profile for eachindividual spot produced by the offset fly lens arrangement.

The TDI CCD sensor provides a relatively high dynamic range for scanningand provides the ability to scan a surface on the order of 500 timesfaster than previous systems. The implementation of the TDI CCD in thepresent system is adjacent to and preceded by a pinhole mask havingsimilar pinhole dimensions as the offset fly lens array. The lensingarrangement, offset fly lens array, and the pinhole mask are arranged toprovide an accurate image on the sensor based on the type of specimenbeing inspected. The offset fly lens array and the pinhole mask aremechanically well aligned to one another during system operation andscanning. Generally, the TDI CCD has an opaque surface, such as metal,located on the back of the ultraviolet TDI, with polysilicon on thefront side of the TDI sensor. The polysilicon is opaque in theultraviolet range.

One potential use of the system is in conjunction with a slit laserconfocal arrangement. Also, the system may employ autofocus capabilitieswhich must be kept stable for particular wafer specimens. Unstableautofocus may tend to cause severe signal variations and is generallyunusable. In particular, the autofocus used in the present systemignores various layers rather than evaluating multiple confocalmeasurements and cancelling undesirable measurements. The process ofignoring layers not pertient to the scan is that focus is more rapid andeffective for multiple height measurements.

If the system uses a large NA (numerical aperture) objective, high angledarkfield can be employed in the system by using an aperture in afourier plane. Confocal Z discrimination, inherent in confocal systems,reduces the necessity for providing both high angle of incidence andgrazing angle dark field evaluation. In such a high angle dark fieldarrangement, the binary fly lens arrangement permits constructingefficient directional dark field spots for confocal imaging.

An alternate arrangement for dark field confocal imaging may use a UBBarc lamp to provide light energy to a beam compressor/expander whichtakes light energy from the multidirectional arc lamp and focuses thelight energy from the arc lamp to create a broad illumination field.This embodiment employs a pinhole array to convert light energy from thebroad illumination field into an offset pattern of illumination spots.The offset pattern of illumination spots is transmitted to a lens whichfocuses the array of light energy toward the surface of the waferspecimen. Light energy then passes through a transmitter/reflector whichtransmits the light energy toward the surface of the wafer specimen.Light energy from the transmitter/reflector passes through an objectivewhich provides an array of beams, rather than a single beam, onto thesurface of the wafer specimen, and the image from the wafer surface isreflected back through the objective. Light energy passes toward thetransmitter/reflector, which reflects light energy toward a Mag tube.Mag tube focuses the multiple beams toward pinhole mask. The pinholemask again must be mechanically aligned with the pinhole array to bewithin good mechanical alignment. Light energy passing through each ofthe pinholes in the pinhole mask is directed toward a relay lens, whichdirects light energy onto a sensor.

An alternate embodiment of the current invention is to use an AODscanning system, which provides better mechanical stability over amechanical polygon scanning system. This is increases the stabilityprovided by the fixed array of conjugate pinholes in the illuminationand collection paths of the system.

Other objects, features, and advantages of the present invention willbecome more apparent from a consideration of the following detaileddescription and from the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical prior art laser confocal microscopyinspection arrangement;

FIG. 2 presents a multilevel specimen which benefits from confocalinspection is a system using parallel processing of brightfield anddarkfield data;

FIG. 3 a is an illustration of three returns from the scan of thespecimen illustrated in FIG. 2;

FIG. 3 b shows a combination of the three scan images of FIG. 3 a;

FIG. 3 c illustrates the signal received by a sensor wherein pinholesare used to block out of focus portions of the signal;

FIG. 4 illustrates the filtering of images using a confocal arrangement;

FIG. 5 presents the inventive system disclosed herein using an offsetfly lens arrangement;

FIG. 6 is a section of the offset fly lens array;

FIG. 7 presents an alternate arrangement of the current system for usewith dark field confocal imaging;

FIG. 8 illustrates a cross section of the topology of a typical waferspecimen; and

FIG. 9 is a schematic of the control system for providing autofocususing cancellation.

DETAILED DESCRIPTION OF THE INVENTION

The inventive system disclosed herein employs multiple wavelengthconfocal inspection techniques in conjunction with an offset fly lensarrangement and a TDI CCD sensor. Multiple wavelength confocalinspection enhances the ability to obtain an extended focus image orenhanced surface profile information with a single scan, or the abilityto scan at varying specimen depths while maintaining focus on thevarious depths during a single scan pass. The use of such techniquesprovides light to be transmitted at multiple wavelengths withoutcrosstalk.

FIG. 2 illustrates a multilevel specimen which benefits from confocalinspection. A first scan frequency provides a focused image based on thetopmost surface 201 of the specimen, while a second scan frequencyprovides a focused image based on a contact bottom 202 of the specimen.A further third scan frequency, not shown, provides a focused imagebased on the bottom surface of the specimen.

The three returns from the scan of the specimen illustrated in FIG. 2 ispresented in FIG. 3 a, where the x coordinate of FIG. 3 a represents thelateral position during the scan and the y coordinate represents thereflected intensity. A system employing confocal inspection for such amultilevel specimen then combines the scan images as shown in FIG. 3 b,which represents the unfiltered return from the arrangement illustratedin FIG. 2. Pinholes are used to block any signal which is out of focus,resulting in the sensor receiving the signal shown in FIG. 3 c.

FIG. 4 illustrates the filtering of images using a confocal arrangementand the operation of the pinholes. The illuminator and sensor (both notshown) are located away from the pinhole 401, focusing lens 402, andwafer specimen 403. The illuminator transmits light through the pinhole401 and toward the objective lens, or focusing lens 402. The focusinglens 402 provides focus of the light beams onto the wafer 403, here withthe focal plane positioned on the leftmost illustrated, or top, surfaceof the wafer 403. Some light is transmitted through the top layer of thewafer as shown, reflecting off an interior layer at a particular angleand being directed toward focusing lens 402. Focusing lens 402 transmitsthe light energy as shown toward the pinhole 401, but as shown the outof focus rays are blocked by the spatial filter or pinhole. The lightenergy outside the pinhole diameter is out of focus, and only thefocused, central light images pass through the pinhole and on toward thesensor.

The inventive system 500 is illustrated in FIG. 5. Laser 501 transmitslight energy toward a beam expander 502 which expands the light energyfrom the laser beam to create an illumination field. The system uses anoffset fly lens array 503 to convert the light energy from theillumination field into an offset pattern of illumination spots. Theoffset pattern of illumination spots is transmitted to a first lens 504which relays the array of light energy produced toward the surface ofthe wafer specimen 505. Light energy then passes through atransmitter/reflector 506 which transmits the light energy toward thesurface 508 of the wafer specimen 505. Light energy from thetransmitter/reflector 506 passes through objective 507 which provides anarray of beams, rather than a single beam, onto the surface 508 of thewafer specimen, and the image from the wafer surface 508 is reflectedback through objective 507. If a surface of specimen 505 is within oneof the spots transmitted thereupon, the radiation is reflected back fromsaid specimen 505 and through the system 500. Light energy passes towardtransmitter/reflector 506, which reflects light energy toward focusingtube 509. Focusing tube 509 focuses the multiple beams toward pinholemask 510. The pinhole mask 510 must be mechanically aligned with theoffset fly lens array 503 as described below. Light energy passingthrough each of the pinholes in the pinhole mask 510 is directed towardrelay lens 511, which directs light energy onto sensor 512. Sensor 512is preferably a TDI sensor, but other sensors may be used which permitreceipt of light energy.

FIG. 6 illustrates a portion of the offset fly lens array 503.Preferably the offset fly lens array 503 is a 16 (X direction) by 128 (Ydirection) element array of lenses. A lens element 601 comprises anexterior edge 602, an interior portion 603, and a central pixel grouping604, with each central pixel grouping 604 comprising 16×16 pixels. Thepinhole mask 16×128 pinhole arrangement, providing a one-to-onecorrespondence between the lens elements and the holes in the pinholemask. The offset pattern of the offset fly lens array 503 is chosen andarranged such that the spots produced from the offset fly lens array 503can be recombined in the system into a continuous image by image sensor512. Binary optics (not shown) may be employed to create a darkfieldillumination profile for each individual spot produced by the offset flylens arrangement 503. Such profiles could include bright fieldillumination, dark field illumination, and/or directional dark fieldillumination. The binary optics are used to construct an inexpensive flylens array having uniform or non uniform illumination profiles which canimprove overall signal to noise ratio of defect detection.

A wafer stage (not shown) affords the system the ability to move thewafer specimen 505 beneath the spots produced to provide the ability toimage any and all portions of the wafer specimen. An imaging opticstrain (not shown) images the spots onto the pinhole mask 510. Thepinhole mask 510 has the same offset pattern and alignment as the offsetfly lens array 503. Preferably the system 500 of FIG. 5 does not includeany moving parts apart from the stage transporting the specimen.

Alternative arrangements to that shown in FIG. 5 may be used. Forexample, the pinhole array may be located on the TDI sensor or beseparate from the sensor and the pinhole array may use a relay lens forconfocal in/out selectability. Generally speaking, the lensingarrangement and the offset fly lens array 503 and the pinhole mask 510must be carefully arranged to provide an accurate image on the sensor.The offset fly lens array 503 and the pinhole mask 510 must be inmechanical alignment with one another during system operation andscanning.

One potential alternate use of the system is in conjunction with a slitlaser confocal arrangement. Also, the system may employ autofocuscapabilities which must be kept stable for particular wafer specimens.Unstable autofocus may tend to cause severe signal variations in thearrangement illustrated in FIG. 5. In the present system, spots or areasof interest on the specimen surface may be located at different heights,i.e. at varying distances from the objective 507. The ability to discernfeatures on specimens having varying heights is provided by thefollowing autofocus system. First, the system receives the informationfrom the area of interest including height measurements and data at theparticular heights. Thus the instrument detects the topological featuresof the specimen as the specimen passes along the path of scanning.

An example of a cross section of a wafer specimen is presented in FIG.8. The cross section of FIG. 8 is typical for a memory device having acentral memory array surrounded by peripheral circuitry such asrow/column decoders. The arrangement disclosed in FIG. 8 represents sucha specimen at an intermediate stage of fabrication, including dielectric801, peripheral circuitry (first metal layer) 802, memory array (secondmetal layer) 803, streets 804, underlying silicon 805, and a backsidearea 806. During scanning, the resultant focus position will vary acrossthe die because the illumination used for focus “sees” the underlyingmetal and silicon topology.

Generally the system applies an offset to the autofocus system toprovide as much in-focus viewing over the entire die area. The currentautofocus system measures variations across a specimen, records thevariations, and cancels the variations during inspection, thusmaintaining a planar focus condition. During scanning, die or specimenrows are swathed with the autofocus system responding to the topologypresent within the swathing field of view. Swaths frequently overlap oneanother to account for variations in topology in paths perpendicular tothe direction of scanning. The system maps the focus profile for theentire specimen, and it is preferable to have a single focus profile.Therefore the system bottom adjusts the swathing to avoid heightvariations in neighboring elements. In operation, the system scans thedie and records the focus profile. Several die may be averaged to removenoise and wafer tilt by equalizing end points of the focus scan.

The addition of an offset is shown in the block diagram of FIG. 9. FIG.9 shows that planar autofocus is effectuated by holding the focusactuator at a static position, wherein the system records the focussignal across each swath of the die. During inspections, the focusprofile is replayed into the autofocus control loop to cancel the effectof the underlying topology. This cancelling aspect provides a planarfocus condition at a height which is not necessarily that of the topsurface of the wafer. The system applies a further DC offset eitherelectronically or optically to select the top surface. In FIG. 9, theoffset is sent into summing element 901, which is applied to amplifierand compensator 902 and to focus actuator 903. A typical system includesan amplifier and compensator and may include an actuator for dynamicfocus. In the present system, focus feedback is provided via focusfeedback system 904, which feeds the focus signal to summing element 901and back to the recording element for recording the depth measurements.

With respect to the pinhole mask 510 used in the current system, theapertures in pinhole mask 510 are preferably the size of or smaller thanthe diffraction limit of the radiation generated. Further, theaperatures are separated from one another by at least several times thediffraction limit to minimize the potential for stray radiation. Thepattern of apertures is selected such that every point of the viewedsurface moves through an illuminated spot corresponding to the spots onthe mask. The view of the spots on the specimen is recorded and therecorded data is processed to generate a complete image of the specimen.Pinhole offset is not necessarily equal to pinhole width, and pinholesmay overlap to provide redundancy.

Images are acquired by scanning the object along the path and reading anoutput from the TDI sensor each time the object moves by a predeterminedwidth, for example the width of one pixel.

If the system uses a large NA (numerical aperture) objective, high angledarkfield can be employed in the system by using an aperture in afourier plane. When high incidence angle darkfield evaluation isemployed in conjunction with a binary fly lens arangement, the binaryfly lens arrangement allows construction of efficient directional darkfield spots for confocal imaging. The presence of confocal Zdiscrimination, inherent in confocal systems, can reduce the need forhigh incidence angle (grazing angle) dark field evaluation.

An alternate arrangement for dark field confocal imaging is presented inFIG. 7. As shown therein, a UBB arc lamp 701 provides light energy to abeam compressor/expander 702 which receives the light energy transmittedby the omnidirectional arc lamp 701 and expands the light energy tocreate a broad illumination field. The system employs a pinhole array703 to convert light energy from the broad illumination field into anoffset pattern of illumination spots. The offset pattern of illuminationspots is transmitted to a lens 704 which focuses the array of lightenergy toward the surface of the wafer specimen 705. Light energy thenpasses through a transmitter/reflector 706 which transmits the lightenergy toward the surface of the wafer specimen 705. Light energy fromthe transmitter/reflector 706 passes through objective 707 whichprovides an array of beams, rather than a single beam, onto the surface708 of the wafer specimen, and the image from the wafer surface 708 isreflected back through objective 707. Light energy passes towardtransmitter/reflector 706, which reflects light energy toward Mag tube709. Mag tube 709 focuses the multiple beams toward pinhole mask 710.The pinhole mask 710 again must be mechanically aligned with the pinholearray 703 to be within plus or minus one micrometer mechanicalalignment. Light energy passing through each of the pinholes in thepinhole mask 710 is directed toward relay lens 711, which directs lightenergy onto sensor 712. Sensor 712 is preferably a TDI sensor, butagain, other sensors may be used which permit receipt of light energy.As with the embodiment illustrated in FIG. 5, an alternate to the relaylens 711 is to place the pinhole mask on the surface of the sensor 712.The TDI sensor assembles the final image based on the array of spots.The result is a wider swath of the image is viewed in a single pass,thereby decreasing throughput without image degradation.

The construction of sensor 712 when employing a TDI CCD sensor is asfollows. First, the pinhole mask 710 is located adjacent the TDI sensor,either directly in contact with the sensor or spaced a short distancefrom the sensor 712. The back of the TDI sensor is constructed of anopaque material, such as a metal, with polysilicon preferably located onthe front side of the detector and forming the pinhole mask. Thepolysilicon is also opaque in the ultraviolet range.

An alternate embodiment of the current invention is to use an AODscanning system, which provides better mechanical stability over amechanical polygon scanning system.

While the invention has been described in connection with specificembodiments thereof, it will be understood that the invention is capableof further modifications. This application is intended to cover anyvariations, uses or adaptations of the invention following, in general,the principles of the invention, and including such departures from thepresent disclosure as come within known and customary practice withinthe art to which the invention pertains.

1-33. (canceled)
 34. A system for inspecting a specimen, comprising: anoffset calculator configured to calculate a baseline focal offset forthe specimen; a focal actuator configured to dynamically set systemfocus partially based on the baseline focal offset; and a summingelement configured to sum dynamically set system focus and baselinefocal offset to produce a focal error; wherein said focal error isemployed in inspecting said specimen.
 35. The system of claim 34,wherein the summing element provides the focal error to the offsetcalculator.
 36. The system of claim 34, wherein said focal actuatorprovides signals through a focus feedback system to a recording device.37. The system of claim 34, further comprising an amplifier andcompensator that receive the focal error, amplify and compensate thefocal error, and provide the amplified and compensated focal error tothe offset calculator.
 38. A method for inspecting a specimen,comprising: calculating a baseline focal offset for specimen inspection;dynamically setting focus parameters at least partially based on thebaseline focal offset; and summing dynamically set focus parameters andbaseline focal offset to produce a focal error; wherein said inspectingof the specimen employs the focal error.
 39. The method of claim 38,wherein summing provides the focal error to an offset calculator. 40.The method of claim 38, wherein dynamically setting comprises providingsignals through a focus feedback system to a recording device.
 41. Themethod of claim 38, further comprising receiving the focal error,amplifying and compensating the focal error, and providing the amplifiedand compensated focal error to the offset calculator.
 42. The method ofclaim 38, further comprising: performing an initial scan to determinespecimen depth features; determining a bottom depth on the specimen andsetting a bottom inspection threshold at the bottom depth; andperforming a feature scan of the specimen using the bottom inspectionthreshold and modulating focus depth based on the initial scan.
 43. Themethod of claim 42, wherein performing the initial scan comprisesperforming multiple inspection swaths across the specimen.
 44. Themethod of claim 43, wherein the specimen comprises a semiconductorwafer.
 45. The method of claim 44, wherein the semiconductor wafercomprises: at least one metal layer; and silicon underlying the at leastone metal layer.
 46. The method of claim 42, wherein said methodmaintains a relatively planar focus condition.
 47. The method of claim42, further comprising scanning additional specimens and equalizing theresultant scans.
 48. The method of claim 47, wherein equalizing theresultant scans comprises averaging specimen values thereby removingnoise and tilt.
 49. The method of claim 42, wherein the inspectionsystem comprises a fly lens arrangement, and performing the initial scanand performing the feature scan employ the fly lens arrangement.
 50. Asystem for inspecting a specimen, comprising: means for calculating abaseline focal offset for the specimen; means for dynamically settingsystem focus partially based on the baseline focal offset; and means forsumming dynamically set system focus and baseline focal offset toproduce a focal error; wherein the specimen is inspected using the focalerror.
 51. The system of claim 50, wherein the summing element providesthe focal error to the offset calculator.
 52. The system of claim 50,wherein said focal actuator provides signals through a focus feedbacksystem to a recording device.
 53. The system of claim 50, furthercomprising an amplifier and compensator that receive the focal error,amplify and compensate the focal error, and provide the amplified andcompensated focal error to the offset calculator.